Displacement pumps deliver fluids at a volume flow which is proportional to the speed of the pump. The delivery volume per revolution or per reciprocal stroke—the so-called specific volume flow—is constant or can at least be regarded as a good approximation of constant in practice. The constancy per revolution or stroke and accordingly the proportionality to the speed of the pump is for example an interference in applications in which the assembly to be supplied by the respective pump has a fluid requirement which is lower in one or more speed range(s) of the pump than the volume flow resulting from the proportionality. In the respective speed range, the pump accordingly delivers a volume flow which is above the requirement and is channeled away with losses. This problem is described in U.S. Pat. No. 6,126,420, which already discloses an internal gear pump exhibiting an adjustable delivery volume for solving the problem.
U.S. Pat. No. 6,244,839 B1 likewise discloses an internal gear pump exhibiting an adjustable delivery volume. For adjusting, the inner toothed wheel can be axially shifted relative to the outer toothed wheel. The inner toothed wheel is part of an axially shifting adjusting unit which is formed as a piston which acts on both sides. Via a 4/3 adjusting valve, the adjusting unit is charged with the fluid which is delivered by the pump. The adjusting valve comprises a valve casing and a valve piston which can be axially moved back and forth within the valve casing and is charged at one axial end with the delivered fluid and at the other axial end with a force of a valve spring which counteracts the pressure of the fluid. The position of the valve piston is set in accordance with the equilibrium of the force of the valve spring and the force generated by the fluid pressure. The adjusting valve is configured such that when a fluid pressure predetermined by the valve spring is reached, the adjusting unit is moved out of an axial position for a maximum delivery volume towards an axial position for a minimum delivery volume. The biasing force of the valve spring is set beforehand at the adjusting valve.
WO 03/058071 A1 discloses a displacement pump comprising an adjusting valve in which the movable valve piston for adjusting the delivery volume of the pump is charged in an axial direction with fluid from the high-pressure side of the pump and charged counter to the fluid with a spring force. In order to be able to lower the fluid pressure which the pump is regulated down to, a control device for the adjusting valve is provided which exerts an additional force on the valve piston. An electric step motor for adjusting the biasing force of the valve spring and a magnetic coil for generating an additional magnetic force may be cited as examples of control devices. The fluid flow connected by means of the adjusting valve acts on a shifting unit of the pump in the direction of a maximum delivery volume only, while the pressure of the high-pressure side constantly acts in the opposite direction.